As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased. Among them is a lithium secondary battery having high energy density and voltage and excellent preservation and service-life characteristics, which has been widely used as an energy source for various electronic products as well as the mobile devices.
Based on their external and internal structures, the secondary batteries are generally classified into a cylindrical battery, a prismatic battery, and a pouch-shaped battery. Especially, the prismatic battery and the pouch-shaped battery, which can be stacked with high integration and have a small width to length ratio, have attracted considerable attention.
An electrochemical cell having a cathode/separator/anode structure, which constitutes a secondary battery, may be generally classified as a jelly-roll (winding) type electrochemical cell or a stacking type electrochemical cell based on the structure of the electrochemical cell. The jelly-roll type electrochemical cell is manufactured by coating a metal foil to be used as a current collector with an electrode active material, drying and pressing the coated metal foil, cutting the dried and pressed metal foil into the form of a band having a predetermined width and length, isolating an anode and a cathode from each other using a separator, and helically winding the anode/separator/cathode structure. The jelly-roll type electrochemical cell is suitable for cylindrical batteries; however, the jelly-roll type electrochemical cell is not suitable for prismatic batteries or pouch-shaped batteries because the electrode active material may be detached, and the spatial utilizability is low. On the other hand, the stacking type electrochemical cell is an electrochemical cell constructed in a structure in which a plurality of cathode and anode unit cells are sequentially stacked one on another. The stacking type electrochemical cell has an advantage in that the stacking type electrochemical cell can be constructed in a prismatic structure; however, the stacking type electrochemical cell has disadvantages in that a process for manufacturing the stacking type electrochemical cell is complicated and troublesome, and, when external impacts are applied to the stacking type electrochemical cell, electrodes of the stacking type electrochemical cell are pushed with the result that short circuits occur in the stacking type electrochemical cell.
In order to solve the above-described problems, there has been developed an electrochemical cell having a novel structure, which is a combination of the jelly-roll type electrochemical cell and the stacking type electrochemical cell, i.e., an electrochemical cell constructed in a structure in which full cells having a cathode/separator/anode structure of a predetermined unit size or bicells having a cathode (anode)/separator/anode (cathode)/separator/cathode (anode) structure of a predetermined unit size are folded using a long continuous separation film. Examples of such an electrochemical cell are disclosed in Korean Unexamined Patent Publication No. 2001-82058, No. 2001-82059, and No. 2001-82060, which have been filed in the name of the applicant of the present patent application.
In the electrochemical cell having the above-described structure, however, the long separation film is used with the result that the electrodes of the unit cells may be separated from the separation film or the electrodes of the unit cells may be twisted due to impacts and vibrations applied to the outside and the inside of the battery, which may cause the occurrence of short circuits in the secondary battery. According to circumstances, the secondary battery may catch fire or explode.